Neuroanatomy refers to descending cranial nerve fibers outside the pyramidal system as extrapyramidal system. These fibers do not pass through the ventral cone of medulla oblongata, and do not directly stop at the α motor neurons of spinal cord to control their motor function; But indirectly affects and regulates the function of spinal cord α motor neurons through the function of intermediate neurons or spinal cord γ motor neurons. The extrapyramidal system is complex in composition, and its fibers come from various structures, including cerebral cortex, striatum, globus pallidus, subthalamic nucleus, substantia nigra, red nucleus and reticular structure of brain stem. In addition, the nerve fibers of cerebellum system can also be regarded as a part of extrapyramidal system.
The fiber connection of the extrapyramidal system is complex, with many downward connections and many reentry connections.
Extrapyramidal system plays an important role in maintaining moderate muscle tension, posture and accuracy of voluntary movement. The movement function of extrapyramidal system is the premise and guarantee of voluntary movement's accuracy. Therefore, the main dyskinesia of extrapyramidal system are abnormal muscle tone and dyskinesia. Abnormal muscle tension is gear-like stiffness. When the doctor pulls the patient's bent limb hard, it feels like pulling a gear and gradually straightening the bent limb intermittently. This kind of gear-like rigidity of limb muscle tension makes patients often bend their legs and arms with two fists and a half, walk in a panic, rush forward and fall. Due to the abnormal facial muscle tension, the patient lacks the conversion of facial expression and presents a mask face. Extrapyramidal dyskinesia is characterized by static tremor, hand and foot peristalsis, torsion spasm and so on. In the clinical work of neurology, extrapyramidal dyskinesia and abnormal muscle tone are often collectively referred to as extrapyramidal symptoms.